My compressor makes me wince every time it charges. My seam is off the bottom, but I really dislike a pressure vessel with a seam aligned with the
worst case stress.
I came very close to building an external house for it. I really will now.
Imagine the barotrauma from that, even if you didn't get taken out by shrapnel.
hmm does look like it got very thin due to rust inside. Yeah I have no idea what the condition of mine are like inside or ever drained water out of
them so think I'll have a look this weekend. That sure looked like a huge explosion
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Building: Dax Rush - very, very slowly....
posted on 18/2/22 at 06:07 PM
quote:Originally posted by adithorp
And that's why my insurance insist on engineers reports. Including ultrasonic thickness checks on the receiver incase of internal corrosion.
same here - insurance company won't touch it without an annual inspection
My compressor lives outside in a B+Q garden storage box, wired and piped inside. Best thing i ever did from noise and freeing up space in my
workshop....and of course if it should go 'pop' it's highly unlikely anyone will be anywhere near it.
quote:Originally posted by adithorp
And that's why my insurance insist on engineers reports. Including ultrasonic thickness checks on the receiver incase of internal corrosion.
You see things like this and makes you wonder if buying a secondhand one is really worth it as you'll never really know what your buying?
I want a bigger one and was thinking of buying used, but not now...
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Building: Dax Rush - very, very slowly....
posted on 18/2/22 at 10:49 PM
quote:Originally posted by perksy
There's some serious rust inside that tank
You'd be amazed how quickly they can rust internally - they just get thinner and thinner until one day they go bang
(I'm not going to break out the thin wall pressure vessel calculations though - it'll give me nightmares, I always hated those calcs...)
after a few years all of them will be thinner at the bottom than the top - the only way to know when its got too thin is to get them tested - plenty
of places will do testing for you so why risk it
My dad's big machine mart one gets tested every year or two (business insurance send there own contractor we get no say)
basically its drained and visually inspected, then thickness checked with UT, then hydro-static pressure test to check the relief valve (filling with
water makes it much safer during the test) - its ~20 years old now and might only have a few more years according to the reports we get.
(you can normally just replace the tank when it comes to that though)
but this compressor was bought to replace an old second hand one that had been made in the 1930's
we did scrap that one cause there was no way we could trust it, but on inspection it was surprisingly good - I guess they went for the brute force
engineering solution back then (ie. just make it really thick then add some more thickness for luck) - but on the other hand it was that heavy that if
it had gone I wouldn't want to even imagine the shrapnel damage it could have done
besides steel grades/strengths were a bit more haphazard back then too....
My compressor came out of the skip at work and the receiver had been cut up (scrapped due to a bad thermal overload switch (£3) on the motor that
nobody diagnosed). I now have the compressor on a small skid and a 230bar oxygen bottle stood next to it, inverted with a stainless pipework sump at
the bottom. If I need extra capacity, I have a 47kg propane bottle that I pipe into it.
Using a compressor at this time of year, it collects a phenomenal amount of water. I'd say one of the best mods you can make to a standard
compressor would be making some pipework up to make it convenient to drain down the receiver. If it's a fiddly screw underneath, you're
never going to bother. With an elbow, a short pipe and a ball valve, it's dead easy. If you stick a sintered silencer on the end, it's not
even a noisy or messy affair, just stick a little pot under it to catch the water.
I've got an aldi one that is probably 15+ years old. I drained the water a week or two ago and it was orange but only a few drops worth - 10 or
20ml maybe (i'm guessing at a volume). Ran it with the drain bung to try and get anything else out, pressurised it to a couple of psi and opened
the drain again, shook it etc and nothing extra came.
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Building: It is an ex-Locost - it has gone to the IOW!
posted on 20/2/22 at 04:13 PM
quote:Originally posted by MikeR
so how do you know if your compressor is safe?
I've got an aldi one that is probably 15+ years old. I drained the water a week or two ago and it was orange but only a few drops worth - 10 or
20ml maybe (i'm guessing at a volume). Ran it with the drain bung to try and get anything else out, pressurised it to a couple of psi and opened
the drain again, shook it etc and nothing extra came.
It's tricky for an amateur - people who run these things for a living have to get them checked periodically for insurance purposes - and for
safety and occupational health inspector reasons of course. The air tank would be scanned with an ultrasonic probe to check the thickness of the
steel - the inspector would know how thick it should be, and how much corrosion is permissible. There may be an internal inspection with a borescope
as well.
After that, all but one of the holes would be plugged, the air tank filled with water, then more water pumped in to a set pressure - the figure of
150% of working max pressure rings a bell. If it survives that without fracturing then the tank is good - until the next inspection. Don't
rely on my guess - it may be 200% for a steel vessel. At that point the safety over-pressure valve would be checked, together with a check that the
pressure gauge is at least in the correct ball park. These would be tested after the max pressure was tested.
Only the hydraulic test is practical in the home workshop, as long as you can get hold of a decently accurate pressure gauge and a small hand-operated
water pump. When I used to have a 5" gauge model steam locomotive, the club's boiler inspectors used to get their gauge properly checked
for accuracy at regular intervals. Mind you, steam boiler explosions are 100 times more nasty than compressed air tank failures, as the volume of
high-pressure steam at say, 100psi, increases by around 500% when the failure happens...
The advantage of hydraulic testing is that, if there is a failure, all that happens is you get wet - as water is near-enough incompressible then
it's only the elasticity of the tank and fittings that increases the pressure. If it fails then all you usually get is a thin stream of water
piddling out somewhere. Usually, when you're testing, it's one of the things that you've made to plug the holes that leaks - very
annoying! I used to have a box of boiler plugs ready for the 2-yearly inspection (2 years for copper boilers, 1 year for steel) and it was always
them that failed during tests.
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Building: It is an ex-Locost - it has gone to the IOW!
posted on 20/2/22 at 04:19 PM
Out of random curiosity - is there any reason why they don't make air compressor tanks out of stainless steel?
Is it a structural thing? For example, work hardening over time, weld fractures, or similar. Or is it simply that mild steel is cheaper and the
manufacturers are penny-pinching? I guess the latter, but there may be a technical reason.
Do industrial air compressors use stainless steel? You know, the places where cost isn't an issue!
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Building: It is an ex-Locost - it has gone to the IOW!
posted on 20/2/22 at 04:34 PM
BTW: If anyone's wondering I have a 10-year-old large-ish compressor in the garage that only gets drained when I remember and, to be perfectly
honest, gets neglected.
Clearly it's time to give it a proper and thorough inspection...
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Building: Dax Rush - very, very slowly....
posted on 20/2/22 at 07:35 PM
quote:Originally posted by David Jenkins
Out of random curiosity - is there any reason why they don't make air compressor tanks out of stainless steel?
Cost I'd imagine, but partly because normal grades stainless can be a pretty crap structural material - they just don't have the near
perfect elastic behavior with a clear yield point like mild steel typical has (and other structural steels too)
same reason that most stainless (and aluminium alloys) make a poor choice as structural bolt material - there is a reason all the production cars use
8.8 or 10.9 bolts and its not cost...
(Also why none of us should be using stainless bolts on critical bits of our cars like suspension, steering, brakes, roll hoops etc...)
I'm definitely no expert on pressure vessels but in other more general structural use mild steel really only has the corrosion issue to deal
with, if you keep the design within its elastic limit fatigue / creep etc.. just aren't issues for most structures
and as a bonus its easy to form and weld too
In my industry we deal with the corrosion problem by galvanising nearly everything - but I would imagine that could cause a number of issues on a
pressure vessel even if you could add enough bung holes for the venting....
The advantage of hydraulic testing is that, if there is a failure, all that happens is you get wet - as water is near-enough incompressible then
it's only the elasticity of the tank and fittings that increases the pressure. If it fails then all you usually get is a thin stream of water
piddling out somewhere.
[Edited on 20/2/22 by David Jenkins]
Similar to hydraulic systems with hydraulic fluid rather than water. A few years back I was part of the team building a new universal test facility
for Airbus. The hydraulic system was really interesting. The operating pressure was 280 bar or over 4000psi and it was tested to over 6000psi. It
passed first time but I asked what would happen if there was a leak and they basically said, not much - as it is tested hydrostatically hardly any
fluid comes out for a big drop in pressure. Apparently flexible high pressure hydraulic hoses are quite dangerous if they fail though as they can
puncture with small holes that give a very fine needle of spray that can inject itself through people!
quote:Originally posted by David Jenkins
BTW: If anyone's wondering I have a 10-year-old large-ish compressor in the garage that only gets drained when I remember and, to be perfectly
honest, gets neglected.
Clearly it's time to give it a proper and thorough inspection...
Mine had been neglected for a bare minimum 6 years - ie as long as we'd been in the current house surprised how little water came out,
however its only used once in a blue moon.
Fully filled with water, a small, water hand-pump and a pressure gauge are all you need to test - I've done that for a boiler at the local model
engineering society. You would need to replace the safety valve with a solid replacement or at least hold it shut.
For those that are wondering they do make compressor tanks from stainless steel and probably
most of you have experienced one, as for hygiene reasons they supply air to your dentists drill.
I’ve made loads in my time and are used in conjunction with oil free compressors that have PTFE
bearings. The ones I made were made from 10” NB sch10 pipe and a couple of pressed end caps.
Welds argon backed, full penetration and x-rayed, and insured and annually tested by Eagle Star.
